Field of the Invention
The present invention is in the technical field of light source. More particularly, the present invention is in the technical field of a special light source that can be used in those applications that requires high luminance, such as projection displays.
Description of the Related Art
Conventional light sources used in projectors are ultra high performance (UHP) lamps, where mercury plays an important role. An environmental-friendly technology is described in U.S. Pat. No. 7,547,114. In this light source structure, shown in FIG. 1 here, a solid-state light source 100 is used to emit excitation light through a focusing system 102, and a rotation phosphor wheel 104, in which several colorful phosphors are coated in different segments, is excited by the excitation light source to generate a light with a defined color sequence. A is the rotation axis of the color wheel. FIG. 4 of U.S. Pat. No. 7,547,114 (not reproduced here) shows this light source used in a single spatial light modulator (SLM) channels display system.
In the technology described in U.S. Pat. No. 7,547,114, the multiple colors are generated sequentially and combined into a white color in the time domain. When this device is used in a multiple spatial light modulator (SLM) channels display system, in which the multiple colored lights need to be spatially modulated simultaneously, the method described in U.S. Pat. No. 7,547,114 will not work. Let's take the example of three digital light processor (DLP) projectors, shown in FIG. 2. Here, the white light from a lamp 201 and reflector 202 is condensed by the lens 203, homogenized by the integration rod 205, and relayed to a TIR prisms plus Philips type prisms 212 by a lens group containing lens 206, 207 and 208. White light is separated to red, green and blue lights by the Philips type prisms and arrive at the corresponding digital micromirror devices (DMD) 209, 210 and 211. Each of the colored light is spatially modulated simultaneously. The modulated colored light is combined together by the Philips type prisms again and sent to a projection lens 213.
Based on the optical architecture of the above three-DLP systems, a light source with a wide spectrum that covers red, green and blue is desired. As shown in FIG. 3, one of the possible ways to provide such a light source is to change the multi-phosphor segments to three separate and individual single color phosphor devices and combine the three colored light together by a color combiner, such as dichroic filters. In FIG. 3, one excitation source 301 excites a red phosphor wheel 303 and generates red color light, another excitation source 305 excites a green phosphor wheel 307 and generates green color light, and the last source 309 excites a blue phosphor wheel 311 and generates blue color light. To obtain the white light, these three colored lights need to be combined in wavelength domain by using a color combiner 313. As seen in FIG. 4, emission from a red phosphor and emission from a green phosphor are both very wide, and the two spectra have a strong overlap in the wavelength domain. A typical color combiner's accepted spectral bandwidth is shown in FIG. 5. It is clear that for red color, this color combiner will reject light that has shorter wavelength than 600 nm. However, from FIG. 4, the emission of red phosphor has significant radiant power below 600 nm. Therefore lights emitted by phosphors for each individual color usually have wider spectral bandwidth than the accepted spectral bandwidth of the color combiner. The result is that the light outside of the accepted spectral bandwidth is rejected and wasted. As a result, the multiple individual single color phosphor wheels method has low efficiency if used in multiple SLM systems including three DLP, three LCD or three LCOS projectors.